The present invention generally relates to dispersive elements and the like, and more specifically to dispersive elements and the like such as transmission gratings used in optical instruments.
Diffraction gratings are used in various optical instruments. Specifically, many diffraction gratings are used in spectrometers, wave filters and the like to utilize sharp dispersion characteristics thereof. Moreover, it is possible to manufacture the diffraction gratings while arbitrarily determining pitches thereof. Accordingly, a diffraction grating is sometimes used as an angle changing element, a light separating and mixing element or the like in an instrument using a light source such as a laser. The application of the diffraction grating also spreads to the field of displays where light is used as a means of expression instead of a means of communication.
There are already numerous basic researches and implementation examples involving the diffraction gratings, which are compiled in “Introduction to diffractive optical elements” (edited by optical design research group, Optical Society of Japan affiliated to The Japan Society of Applied Physics, published by The Optronics, Co., Ltd.) or the like, for example. To enhance diffraction efficiency, transmission gratings include a blazed type, a binary type, and the like.
FIG. 11 is a view for explaining a configuration of a conventionally used blazed-type diffraction grating. The blazed-type diffraction grating is designed to have a triangular cross-sectional shape with two straight lines so as to enhance the diffraction efficiency for a specific wavelength. That is, as shown in FIG. 11, the surface shape of the blazed-type diffraction grating is formed into grooves with triangular cross-sectional shapes (saw-tooth shapes), and a base angle of this triangle is equivalent to a blaze angle.
Meanwhile, an unillustrated binary-type diffraction grating includes finely arranged rectangles with different levels in terms of cross-sectional shapes. Such a shape is formed by lithography or electron beam drawing.
In the meantime, instead of the diffraction grating, there is also an element formed by arranging metal thin lines as polarizers (“Basic optics”, co-authored by Keiei Kudo and Fumiya Uehara, Gendai Kogaku Sha). This element is formed by arranging metal thin lines such as wires evenly in a lattice and is configured to utilize functions of absorbing or reflecting parallel polarized components with respect to the thin lines and to transmit only perpendicular polarized light with respect to the thin lines. This element is normally referred to as a wire grid polarizer. In general, the thin lines can be formed at intervals of 1 μm at the smallest. Accordingly, the use of the wire grid polarizer is limited to a linear polarizer for infrared rays, for example. Based on the same principle, there is also provided a type of a reflection grating called an echelette grating with shallow blaze angles, in which grating constant is reduced by forming metal thin lines thereon by means of oblique evaporation of metal.
FIG. 12 is a view showing an example of the wire grid polarizer. Here, the shape of the echelette grating as the reflection grating is utilized to form the metal thin lines on ridge portions by oblique evaporation of the metal. By narrowing the grating constant, the wire-grid polarizer is used as a linear polarizer in the range from near infrared rays to visible light. In such a polarization element, the widths of the metal thin lines are made as small as possible by means of evaporating the metal only on the ridge portions of the diffraction grating having triangular cross sections. This is because an increase in the width of the metal thin line causes a decline in efficiency as the polarizer due to an increase in intensity of first-order diffracted light. In general, the polarizer does not function properly unless the intervals of the thin lines are set to one-tenth or below of a wavelength and the widths thereof are set to one-hundredth of the wavelength. Since an element of this kind is used in the form of vertical incidence and transmission with respect to the surface thereof, the original function as the echelette grating is also restrained.
In the meantime, among the related art disclosed in patent publications, there is a technique adopted as a method of manufacturing a diffraction grating for use in photoresist, in which a metal-deposited pattern is formed only on one oblique surface of a diffraction grating, for example (see Patent Document 1, for example). Moreover, there is also disclosed a technique including the steps of forming a resist film only on one oblique surface by oblique evaporation, etching, and then removing the obliquely evaporated resist film (see Patent Document 2, for example).
[Patent Document 1]
Japanese Unexamined Patent Publication No. 63 (1988)-71851 (Page 3, FIG. 1)
[Patent Document 2]
Japanese Unexamined Patent Publication No. 59 (1984)-210403 (Pages 2 to 3, FIG. 1)
Now, in the above-described diffraction grating of the binary type, it is necessary to finely arrange rectangles with different levels in terms of cross-sectional shapes. Accordingly, there are numerous manufacturing processes, and an obtained diffraction grating is very expensive. On the other hand, the blazed type can be manufactured by molding or resin molding through press work as long as a mould is prepared. Accordingly, the blazed type is less expensive and excellent in mass production capability. However, first-order diffraction efficiency of a transmission grating of the blazed type is limited to about 20% at the maximum. Accordingly, when the efficiency is raised for a specific wavelength, a trouble arises in a device dealing with a multi-color light source such as a display because the efficiency of other wavelengths is reduced. Therefore, when the transmission grating of the blazed type is used as a diffraction element, it is necessary to raise the diffraction efficiency and particularly to reduce zero-order transmitted light.
That is, when the transmission grating is used, the zero-order diffracted light, i.e. the directly transmitted incident light inevitably occurs. However, the zero-order diffracted light not only deteriorates utilization efficiency of the light but also incurs stray light for other optical devices located in the vicinity, and thereby causing a problem in terms of a device layout and accuracy. Accordingly, to enhance the diffraction efficiency, it is effective to reduce the zero-order transmitted light.
In the meantime, to enhance the diffraction efficiency, it is necessary to form a diffraction grating member of the diffraction grating into an optimal shape. However, such an optimal shape is complicated and delicate. Accordingly, it is necessary to use electron beam drawing, lithographic technology, and the like upon formation. Since a small size is sufficient for use in the field of optical communication or the like, such a process is relatively easy. However, for use in a display or a projector, the size of the diffraction grating member needs to be as large as several centimeters to 30 centimeters each. Such a diffraction grating member is difficult to process, or even if processed, applicable process costs will be enormous. For this reason, there is an increasing demand for a diffraction grating in the field of a display system, for example, which has a large size, capability of formation at low costs, high diffraction efficiency, and a performance to reduce the zero-order transmitted light in particular. Such a demand is high especially in the field of a liquid crystal display which does not apply a color filter, and the like.
The wire grid polarizer using the echelette grating shown in FIG. 12 is not an application as the diffraction grating in a dispersive element for selecting a specific wavelength or the like, but is merely an application as the polarizer for absorbing/reflecting the polarized components parallel to the thin lines. Accordingly, the metal is evaporated only on the ridge portions of the asperities. It is not possible to reduce the zero-order diffracted light and to enhance the diffraction efficiency when this element is used as the diffraction grating.
Meanwhile, although the oblique evaporation is performed in the technique according to Patent Document 2, the film formed by the oblique evaporation is a resist film and the evaporated film is removed in a finished product. For this reason, no metal film is formed on the produced diffraction grating. Therefore, it is not possible to achieve enhancement of the diffraction efficiency by reducing the zero-order transmitted light in the diffraction grating as the dispersive element.
Moreover, although the metal is obliquely evaporated according to Patent Document 1, the technique of Patent Document 1 is intended to be used as photoresist but is not designed for use in the diffraction grating as the dispersive element. For this reason, the technique according to Patent Document 1 considers a case where a contrast ratio of a projection pattern of the diffraction grating becomes largest, and therefore a semitransparent film is selected as the metal film to be obliquely evaporated so that an intensity ratio between zero-order diffracted light and first-order diffracted light becomes 1 to 1 ratio. To be more precise, metal such as chromium oxide is thinly evaporated in a thickness from 10 to 100 nm. Therefore, even if the technique according to Patent Document 1 is adopted, it is not possible to achieve enhancement of the diffraction efficiency by reducing the zero-order diffracted light. Accordingly, this technique has a difficulty in achieving a performance required in recent years as the diffraction grating to be used in the dispersive element.